TWO-DIMENSIONAL STABILITY AND SEISMIC LOADING MODELS OF CRATER LAKE OUTLET, MOUNT RUAPEHU, NEW ZEALAND

Between major eruptions of Mt. Ruapehu, New Zealand, the vent fills with water, forming the Crater Lake. In the past the Crater Lake’s waterfall outlet has failed, unleashing a catastrophic dam break lahar. Two-dimensional geomechanical modeling of the Crater Lake outlet enables assessment of the stability of the outlet under seismic loading in order to analyze the risk of outlet failure. The natural dam that forms the outlet consists of three different rock units: two strong andesitic lava units and a weaker autoclastic breccia unit. The lava units are modeled using data obtained from laboratory strength testing by previous workers, while the highly heterogeneous breccia unit is modeled using the weighted average approach. Finite element analysis using Phase2 by Rocscience is performed to develop a valid model of the outlet. Peak ground acceleration values ranging from 0-0.5g are applied to the model to predict how the outlet will behave mechanically in the event of seismic activity, both due to regional seismicity and volcano-tectonic quakes associated with Ruapehu. The modeling indicates negligible (millimeter-scale) displacements and ultimately predicts that the outlet will remain stable, even in the event of abnormally high seismicity. This study concludes that seismicity alone is not likely to trigger outlet failure and a subsequent lahar at Mt. Ruapehu.